According to the prior art, polyorganosiloxanes are prepared by hydrolysis and condensation proceeding from methylchlorohydrosilanes with mixed substitution. A direct hydrolytic condensation of hydrogen-containing silanes, for example dimethylmonochlorosilane or methyldichlorosilane, is described, for example, in U.S. Pat. No. 2,758,124. In this prior art method, the siloxane phase which separates during the course of hydrolysis is removed from the water phase comprising hydrochloric acid. Since this process is prone to gelation of the hydrosiloxanes, DE 11 25 180 describes an improved process utilizing an organic auxiliary phase in which the hydrosiloxane formed is dissolved as a separate phase in an organic solvent and, after removal from the acidic water phase and removal of the solvent by distillation, is resistant to gelation. A further process improvement with regard to minimized use of solvent is described by EP 0 967 236. This prior art reference discloses that at first only small amounts of water should be used in the hydrolytic condensation of the organochlorosilanes, such that, in the first step, hydrogen chloride is driven out in gaseous form and can be supplied as a material of value directly for further uses.
Branched organomodified polysiloxanes can be described by a multitude of structures. Generally, a distinction has to be drawn between a branch or crosslink which is introduced via the organic substituents, and a branch or crosslink within the silicone chain. Organic crosslinkers for forming siloxane skeletons bearing SiH groups are, for example, α,ω-unsaturated diolefins, divinyl compounds or diallyl compounds, as described, for example, in U.S. Pat. No. 6,730,749 or EP 0 381 318. This crosslinking by platinum-catalyzed hydrosilylation, which follows downstream of the equilibration, means an additional process step in which both intramolecular bond formation and intermolecular bond formation can take place. The product properties are additionally strongly influenced by the different reactivities of the low molecular weight organic difunctional compounds which tend to form peroxide.
Multiple crosslinking of the silicone block of an organomodified polysiloxane with the organic block copolymer can be effected in various ways. EP 0 675 151 describes the preparation of a polyethersiloxane by hydrosilylation of a hydrosiloxane with a deficiency of hydroxy-functional allyl polyether, in which unconverted SiH functions are bonded with addition of sodium methoxide to the hydroxyl groups of the polyether substituents via an SiOC bond. The increase in molar mass leads to a wide scatter in the product properties, for example the viscosity. A similar approach to the formation of branched systems is described by U.S. Pat. No. 4,631,208, in which hydroxy-functional polyethersiloxanes are crosslinked by means of trialkoxysilanes. The two methods lead to intermolecular crosslinking of the polyethersiloxanes with difficult control of the increase in molar mass and associated, unforeseeable viscosity rises. When the aforementioned prior art methods are pursued, branching within the siloxane moiety at constant molar mass is not obtained, but rather crosslinking to form macromolecular multiblock copolymers is obtained.
Branching within the siloxane chain therefore has to be effected as early as in the preparation of the hydrosiloxane, in order to avoid the described disadvantages of crosslinking. Branches within the siloxane chain require the synthetic incorporation of trifunctional silanes, for example trichlorosilanes or trialkoxysilanes.
As is known to those skilled in the art, the hydrolysis rate of the organochlorosilanes rises in the following sequence (C. Eaborn, Organosilicon Compounds, Butterworths Scientific Publications, London 1960, p. 179)SiCl4>RSiCl3>>R2SiCl2>R3SiCl.
In the hydrolysis and condensation reactions of tetra- and trichlorosilanes, there is therefore an increased tendency to form highly crosslinked gels compared to the slower hydrolysis and condensation reactions of difunctional and monofunctional organochlorosilanes. The established processes for hydrolysis and condensation of dichloro- and monochlorosilanes are therefore not immediately applicable to tetra- and trichlorosilanes, and it is instead necessary to take alternative routes via multistage processes.
Building on this finding, the preparation of monobranched hydrosiloxanes by incorporation of not more than one trifunctional monomer per siloxane chain also has to be performed in two stages according to the prior art. In a first step, a trifunctional, low molecular weight hydrosiloxane is prepared by hydrolysis and condensation from 1,1,3,3-tetramethyldisiloxane and methyltriethoxysilane, as taught, for example, by DE 37 16 372. Only in a second step can an equilibration with cyclic siloxanes to higher molecular weights be effected, as stated by DE 10 2005 004676. For further reaction—and therefore not until in a third step—the monobranched hydrosiloxane thus prepared can be provided with organic substituents by methods known per se for functionalization of siloxane compounds having SiH groups.
For the synthesis of polybranched hydrosiloxanes which, by definition, have more than one tetrafunctional or trifunctional monomer per siloxane chain, two-stage syntheses can also be found in the prior art.
One possibility, described in U.S. Pat. No. 6,790,451, consists in the preparation of a copolymer from trichloromethylsilane or trialkoxymethylsilane with hexamethyldisiloxane or trimethylchlorosilane, also referred to there as MT polymer, which is equilibrated in a second step together with a polydimethyl-(methylhydro)siloxane copolymer. The preparation of such MT polymers requires the use of strong bases or strong acids, in some cases in combination with high reaction temperatures, and gives rise to prepolymers of such high viscosity that their neutralization is considerably hindered, and thus further processing to end products of constant composition and quality is significantly restricted.
According to EP 0 675 151, the hydrolysis and condensation of the SiH-free, branched silicone polymer is first performed in xylene, and, in a second step, the equilibration with methylhydropolysiloxane leads to the branched hydrosiloxane. Here too, two process steps are absolutely necessary, in which the SiH functions are not introduced until the second step.
EP 0 610 818 B1 describes a process for preparing SiH-functional silicone resins proceeding from tetramethyldisiloxane and tetraalkoxysilanes, which are hydrolyzed and condensed using considerable amounts (for example 48 percent by weight of the reaction mixture) of an aqueous-alcoholic hydrochloric acid solution which contains at least 30% by weight of an alcohol and at least 5% by weight of an inorganic acid. The SiH-functional silicone resin thus obtained has to be isolated by extraction with an organic solvent.
EP 1 010 714 B1 describes a method for preparing branched SiH-functional solid silicone resins, in which the solvent used is a high-boiling mixture of alkanes. Although the amounts of water and acidic equilibration catalyst used are smaller than in EP 0 610 818 B1, degradation of the SiH functions used is unavoidable by this prior art method. As Examples 2 and 3 adduced there show, between 10 and 20 mol % of the dimethylhydrosiloxy units used are degraded during the reaction to non-SiH-functional dimethylsiloxy units.
EP 1 050 553 A1 describes a method for preparing branched SiH-functional crosslinkers, in which methyltriethoxysilane, tetramethyldihydrosiloxane and dimethylhydrochlorosilane react with one another in the presence of approx. 20 percent by weight of water, in which the hydrochloric acid released in the hydrolysis of the chlorosilane functions as a Brønsted-acidic equilibration catalyst. As disclosed by Example 1 adduced there, less than 50% of the SiH equivalents used are present in the product.
According to EP 1 829 524, polybranched SiH-functional organopolysiloxanes are used for surface treatment of cosmetic powders. The organopolysiloxanes described there are prepared proceeding from alkoxysilanes and SiH-functional siloxanes by hydrolysis and condensation. The catalyst used is 5 percent by weight of concentrated sulphuric acid. The high amount of acid which, remains in the product, would lead to storage instability and gelation, has to be removed by washing with water after the reaction has ended. This step likewise includes a phase separation and leads to the occurrence of acidic wastewater which has to be disposed of.